1. Field of the Invention
This invention relates to an edge light panel element for a surface light source device used in various illuminating apparatus such as a back light unit for liquid crystal display devices, illuminated display devices, illumination signboards, and the like.
2. Prior Art
A surface light source device of this type normally comprises an edge light panel having a thickness of several (typically from five to six) millimeters. The device has at least one light source, such as a fluorescent lamp, juxtaposed with a side of the panel to permit light issued from the lamp to enter the panel through the side, so that the panel may be evenly illuminated with the light at a intensity of high brightness.
The edge light panel normally comprises a transparent resin substrate which is made of material excellent in light transmittivity, such as an acrylic resin or the like. The panel has its surface so finished as to carry thereon a layer providing a mesh pattern. The pattern may be produced through a screen printing process using a white or pale ink containing white pigments such as titanium oxide or the like, so that the pattern enables the panel to reflect the light irregularly in such a manner that the panel is evenly illuminated with the light through the entire surface thereof. In other words, the light issued from the light source has its intensity vary continuously so as to be represented as a function of a measurement point on the surface of the panel. It is also possible to finish the surface of the substrate of the panel so as to have the surface carry thereon a layer providing a fine dotted pattern or a linear pattern, which pattern may be produced through a blasting process or an appropriate molding process. In any case, the underlying idea is that the light incident on an edge (i.e., the side) of the edge light panel is irregularly reflected or scattered by the surface layer so as to effectively enter the panel and be transmitted as far away as possible within the panel, so that the panel may be evenly illuminated by the light through the entire surface thereof, which enables the panel to improve the display device in brightness and uniformity of display clarity.
Each of the remaining sides of the transparent resin substrate, which sides are not subjected to the incident light, is backed with a reflective tape excellent in reflectivity. As disclosed in Japanese Patent Application No. Hei 2-46419, each of the remaining sides of the substrate is mirror-finished. The side on which the light is incident is so finished as to be smooth and reflective, whereby any faults are removed from the side.
In recent years, surface light source devices of this type described above have been widely used in built-in liquid crystal back light units of wrap-top type personal computers and word processors, and the demand for such devices has been growing rapidly.
A surface light source device for these applications should be very compact and thin and, more specifically, it is often required to have a thickness of less than that of any existing devices, or a thickness of from 2 to 3 millimeters, or in some cases, to have a thickness of 1.5 millimeters.
As the thickness of the edge light panel is reduced, uniformity in brightness of the panel surface becomes poor. This is due to the fact that the incident light issued from the primary light source is apt to be transmitted in an oblique direction along the inner surface of the panel, and particularly to the fact that the primary light source, such as fluorescent lamps or cold cathode ray tubes, is not necessarily coincident in length with the side of the edge light panel to which side the light source is juxtaposed, and the fact that the primary light source is poor in brightness near its opposite ends. The net result of these and other problems may be a surface light source device whose brightness is differs between locations on the surface of the panel, such as the central raised area and the lateral and front sides of the raised area.
In view of these circumstances, it is the object of the present invention to provide an edge light panel which is free from unevenness in brightness particularly in the vicinity of the primary light source, to enable the panel to realize uniformity in brightness to a maximum extent.